1. Field of the Invention
This invention is directed to interfaces between local area networks which in turn each interconnect a number of digital computers or microprocessors. More particularly it relates to interface modules which interconnect electrical and optical local area networks to each other and to their controllers, and which allow bi-directional signal transmission between all devices and channels connected to the interface while incorporating means for preventing endless repetition of signals in the system.
2. Background Information
Local area networks (LANs) are used to link a number of digital computers or microprocessors together for sharing common data, and in some instances, the division of tasks. The network includes a common data highway to which all of the computing devices are connected by controllers so that all the shared data are broadcast to all of the computers and microprocessors on the network. The data may be transmitted on the data highway in the form of electrical or optical signals. In the former case, each of the controllers is connected to a common coaxial cable. Where optical signals are used, an optical coupler device such as a transmissive star coupler, which distributes an optical signal received on any one optical fiber to all other optical fibers, forms the data highway.
Since all of the transmissions are broadcast to all of the computing devices on the network, it is necessary to provide means for precluding interference and the consequent loss of information which would result from simultaneous transmissions on the network. It is also important to establish standards so that the components of various manufacturers are compatible. Accordingly, several protocols have been established for standardizing networks. These protocols address the network on several levels in addition to the physical or hardware level, including a software level.
There are two major approaches to solving the problem of simultaneous broadcasts on a network. One approach allows the individual stations to broadcast when they have data to transmit if the network is clear. Because of propagation delays in the network, it is possible that two stations will start broadcasting almost at the same instant. Hence, once transmission is begun, the transmitting station listens for a collision with another transmission. If a collision is detected, broadcasting is terminated, and reinitiated after waiting a random interval of time. Due to the variability of optical sources and the high rate of attenuation in optical systems, the above method of collision detection is not very reliable on an optical network. One solution, uses optical receivers and logic circuits at the star to determine when two stations are broadcasting simultaneously, and distributes a collision sensed signal to all stations over the star. In response to the collision sensed signal, the stations terminate broadcasting for random intervals before reinitiating transmission.
The second major approach to eliminating interference between transmissions on a network only permits one station to transmit at a time, with a software token passed between the stations designating the active station.
Due to limitations on the physical size of local area networks imposed by such parameters as propagation time, and on optical networks, limited optical transmitter power and high attenuation, it is desirable in some instances to interconnect two or more networks. In addition, Optical LANs are preferred for use in many instrumentation applications, such as for example the protection and control systems for nuclear reactors. The primary reason for the use of optical LANs in such applications is the electrical isolation and the freedom from electromagnetic interference inherent in the dielectric nature of the interconnecting glass fibers. Optical transmitters and receivers, however, are more expensive than electrical transmitters and receivers, and therefore it would be desirable to be able to interconnect electrically those LAN nodes not requiring the electrical isolation or freedom from interference available with the optical channel.
Accordingly, it is a primary object of the invention to provide apparatus which can interface optical and electrical nodes in a local area network.
It is another object of the invention to provide apparatus which can function as an expander to increase the number of optical or electrical LANs nodes in a network.
It is another object of the invention to provide apparatus which can interface optical and electrical LANs.
It is still another object of the invention to provide the above apparatus which allows bi-directional signal transmission between all the devices and channels connected to the apparatus while preventing endless messages repetition and interference.
It is yet another object of the invention to provide a versatile interface module which can satisfy the above objects and can interface LAN controllers to either or both optical or electrical LANs.